The following disclosure relates to switching power supply electrical circuit and signal process.
Switching power supply is used to power many types of electronic devices, for example, lamps. A conventional power supply typically includes a converter. A converter is a power supply switching circuit to convert AC or DC input voltage to a DC output voltage at predetermined value with feedback.
FIG. 1 shows a conventional switching power supply 100 block diagram. In one implementation, Power source 101 is an AC (alternating current) voltage source. Filter 111 prevent high frequency current from entering power source 101. Protection 112 cut off connection between filter and the following circuit at over current condition. Bridge rectifier 113 converts AC sinusoidal waveform to DC waveform. Converter 102 receives a rectified DC voltage from the bridge rectifier 113 and converts to a DC voltage to an output device 103. Converter 102 is a switching power supply electrical circuit. The voltage feedback 104 samples the voltage signal of output device 103 and feeds the signal proportional to lamp voltage back to controller 110 and compare with reference 105 through Opamp 106. Reference 105 is an electrical circuit that supplies very accurate DC voltage. The comparing error signal on output of Opamp 106 will be sent to Comparator 108 and will be compared with Sawtooth Generator 107. The pulse output of Comparator 108 will be sent to Driver 109. The output of Driver 109 will control on time or frequency of main switch of converter 102 to keep output voltage constant at predetermined value set by reference 105.
FIG. 2 shows one implementation of a conventional switching power supply 100.
Power source 101 is an AC (alternative current) voltage source.
In an implementation in which filter 111 includes an Inductor L1 and a resistor R1.
In an implementation in which protection 112 includes a Fuse F1.
Bridge rectifier 113 includes diodes D1, D2, D3 and D4 that rectify AC voltage from Vin to a DC voltage across capacitor C1.
A converter 102 includes a flyback converter circuit, input sense, filter, biasing circuit and snubber circuit. The flyback converter circuit includes a flyback topology converter that receives a DC (direct current) voltage from bridge rectifier 113. Transformer T1, Mosfet M1, diode D8 and capacitor C6 constitute a flyback topology that converts a rectified AC or DC voltage to a constant output DC voltage at predetermined value. Resistors R2, R3 and capacitor C3 compose an input sense circuit. Capacitor C5 is a filter for feedback signal sent to Vsense pin of IC controller. Auxiliary winding T1B, diode D7, zenor diode D6 and capacitor C4 constitute a biasing circuit for IC controller. Capacitor C2, diode D5 and resistor R4 become a snubber circuit to clamp voltage across drain-source of Mosfet M1.Output device 103 is a lamp.Voltage feedback circuit 104 includes an auxiliary winding T1B and resistors R7, R8.Controller 110 is an IC controller that has pins as Vcc, Vin, Vsense, GND and output. The IC controller has comparator 108, Sawtooth Generator 107, Opamp 106, reference 105 and driver 109 integrated inside the controller
In one implementation, for example, the lamp is a LED lamplight emitting diode). The current is approximately an exponential function of voltage.
I=IS(eVD/(nVT)−1); Where, I is the diode current, IS is the reverse bias saturation current, VD is the voltage across the diode, VT is the thermal voltage, n is the ideality factor, and the ideality factor n varies from 1 to 2.
FIG. 3 shows the current with respect of voltage of LED lamp. When voltage has a very little change, current will have a huge change.
Because there are long wires between output of converter and lamp, there is a voltage drop between converter output and lamp. Some factors affect voltage variation. Those are reference tolerance, output voltage sampling, feedback tolerance and voltage drop tolerance across converter output and lamp. Total tolerance will cause voltage variation on lamp. For LED lamp, current will have a huge change when voltage has a very little change, Brightness is proportional to power of LED lamp. Power P=V*I, So conventional switching power supply will cause huge brightness variation for LED lamp. That will cause LED lamp flickering that brings fatigue or hurt to eyes for the long term.